Lithium secondary cell with high charge and discharge rate capability

ABSTRACT

A high capacity, high charge rate lithium secondary cell includes a high capacity lithium-containing positive electrode in electronic contact with a positive electrode current collector, said current collector in electrical connection with an external circuit, a high capacity negative electrode in electronic contact with a negative electrode current collector, said current collector in electrical connection with an external circuit, a separator positioned between and in ionic contact with the cathode and the anode, and an electrolyte in ionic contact with the positive and negative electrodes, wherein the total area specific impedance for the cell and the relative area specific impedances for the positive and negative electrodes are such that, during charging at greater than or equal to 4C, the negative electrode potential is above the potential of metallic lithium. The current capacity per unit area of the positive and negative electrodes each are at least 3 mA-h/cm 2 , the total area specific impedance for the cell is less than about 20 Ω-cm 2 , and the positive electrode has an area specific impedance r 1  and the negative electrode has an area specific impedance r 2 , and wherein the ratio of r 1  to r 2  is at least about 10.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to applicationSer. No. 60/542,550, filed Feb. 6, 2004, entitled “Non-AqueousElectrolyte Secondary Cell with High Charge and Discharge RateCapability,” which is hereby incorporated in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a non-aqueous electrolyte secondary cell. Inparticular, the invention relates to a battery having a fast charge anddischarge rate capability and low rate of capacity fade during such highrate cycling.

2. Description of the Prior Art

Contemporary portable electronic appliances rely almost exclusively onrechargeable Li-ion batteries as the source of power. This has spurred acontinuing effort to increase their energy storage capability, powercapabilities, cycle life and safety characteristics, and decrease theircost. Lithium-ion battery or lithium ion cell refers to a rechargeablebattery having an anode capable of storing a substantial amount oflithium at a lithium chemical potential above that of lithium metal.

Historically, non-aqueous secondary (rechargeable) cells using metalliclithium or its alloys as the negative electrode were the firstrechargeable cells capable of generating high voltages and having highenergy density. However, early on it became clear that their capacitydecreased rapidly during cycling, and that their reliability and safetywere impaired by the growth of the so-called mossy lithium and lithiumdendrites to a degree that precluded these cells from the consumermarket. Importantly, the few lithium-metal rechargeable batteries which,from time to time, were being actively marketed, were recommended to becharged at a rate no higher than ca. C/10 (10-hour) rate to minimize thedendritic growth.

To counteract the slow but unavoidable reaction of lithium with theelectrolyte components, these early cells typically contained a 4–5times excess of metallic lithium as compared with the capacity of thepositive active material. Thus, the observed capacity fade duringcycling was caused by a decrease in the specific capacity of thepositive active material. An up-to-date review of lithium-metalbatteries is available (D. Aurbach et al., Journal of ElectrochemicalSociety, 147(4) 1274–9 (2000)).

To overcome the difficulties associated with the use of lithium metalnegative electrodes, several major improvements in battery materialswere introduced. Various types of carbon capable of highly efficient andreversible intercalation of lithium at low potentials were used as thenegative electrode to eliminate the growth of lithium dendrites. See,U.S. Pat. Nos. 4,423,125 and 4,615,959. Highly conductive liquidelectrolytes have been developed, which are stable at both low and highpotentials vs. lithium. See, U.S. Pat. No. 4,957,833. High-voltage,high-capacity positive electrode materials based on lithiated transitionmetal oxides, such as LiCoO₂, LiMn₂O₄ and LiNiO₂ have been developed.See, U.S. Pat. No. 4,302,518.

Since the electrochemical potential of lithium metal is only ca. 0.1 Vlower than the potential of the fully lithiated graphitic carbonelectrodes, LiC₆, used in Li-ion batteries, both are strongly reducingtowards any materials in contact with them, such as the polymer binderand the liquid electrolyte lithium salt solution. In particular, liquidelectrolyte components react with both metallic lithium and lithiatedcarbon to form a metastable protective layer on the surface of thenegative electrode materials, the so-called solid-electrolyte interface(SEI) (E. Peled, “Lithium Stability and Film Formation in Organic andInorganic Electrolyte for Lithium Battery Systems”, in “LithiumBatteries”, J.-P. Gabano, Ed., Academic Press, London, 1983; p. 43).

However, the process of SEI formation and its partial renewal duringbattery cycling and storage irreversibly consumes a fraction of theactive lithium from the battery and results in a loss of capacity. Thisloss is readily visible when one compares the amount of charge usedduring the first charge and then the discharge of the battery, aso-called formation cycle. During the first charge cycle of a new Li-ionbattery, the positive active material is oxidized and Li⁺ ions diffusein the liquid electrolyte towards the carbon negative electrode, wherethey are reduced to Li⁰ and intercalated between the graphene layers ofthe carbon structure. A fraction of this first-reduced lithium, up toca. 50%, but more typically between 5 and 15% of the intercalatablelithium, reacts to form the above-mentioned SEI. Clearly, the amount ofLi available in the positive electrode material has to be less than thesum of lithium necessary for the formation of the SEI and the availablelithium intercalation capacity of the carbon material. If the amount oflithium removed from the positive electrode material is greater thanthat sum, the excess lithium will be deposited, or plated, as metalliclithium on the external surfaces of the carbon particles. The platedlithium is in the form of a very reactive high-surface-area deposit,so-called ‘mossy lithium’, which will not only degrade the batteryperformance due to its high electrical impedance, but will alsoseriously compromise its safety.

Even if the lithium intercalation capacity of the carbon material islarge enough to accommodate all of the lithium from the positiveelectrode material, it is possible to plate lithium if the charging isdone too quickly.

Due to the strong possibility of lithium plating on the carbon anodeduring the high-rate charge, manufacturers of Li-ion batteries recommendthat such batteries are charged at an equivalent current no greater thanone time the nominal cell capacity (1C) until the upper maximum chargingvoltage is reached, followed by a constant-current (taper) segment(http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_LiIon_Charging.pdf).In practice, the charging step lasts from 1.5 to 2.5 hours, which is toolong for certain applications, such as battery-powered tools, certainelectronic devices and electric vehicles.

It is the object of the present invention to provide a Li-ion batterycapable of high charge and discharge rates, inexpensive to make, safeduring extended high-electrical-stress use, having high energy and powercapability, and exhibiting low capacity and discharge power loss afternumerous high-rate charge and discharge cycles.

SUMMARY OF THE INVENTION

In one aspect, a secondary cell and secondary cell manufacturing andcycling methods that are useful in high-rate applications are provided.The positive lithium storage electrode and the negative electrode areboth capable of reversibly intercalating lithium at a high rate. Thecell does not plate lithium during charging, resulting in reducedcapacity fade over many charge cycles. Thus, the high-performancelithium-ion cell is capable of repeated, safe and stable charge anddischarge at exceptionally high rates of charge and discharge. Forexample, such a battery can be charged at 10C rate and discharged at 20Crate, with a capacity loss as little as 0.008% per cycle over more than1,000 cycles. In addition, the secondary cell can achieve up to 95%state of charge in as little as six minutes.

In one aspect of the invention, a high capacity, high charge ratelithium secondary cell is provide, which includes a high capacitylithium-containing positive electrode in electronic contact with apositive electrode current collector, the current collector inelectrical connection with an external circuit, a high capacity negativeelectrode in electronic contact with a negative electrode currentcollector, the current collector in electrical connection with anexternal circuit, a separator positioned between and in ionic contactwith the cathode and the anode, and an electrolyte in ionic contact withthe positive and negative electrodes, wherein the total area specificimpedance for the cell and the relative area specific impedances for thepositive and negative electrodes are such that, during charging atgreater than or equal to 4C, the negative electrode potential is abovethe potential of metallic lithium.

In another aspect of the invention, a high capacity, high charge ratelithium secondary cell includes a lithium-containing positive electrodein electronic contact with a positive electrode current collector, thecurrent collector in electrical connection with an external circuit, anegative electrode in electronic contact with a negative electrodecurrent collector, the current collector in electrical connection withan external circuit, a separator positioned between and in ionic contactwith the cathode and the anode, and an electrolyte in ionic contact withthe positive and negative electrodes, wherein the charge capacity perunit area of the positive and negative electrodes each are at least 0.75mA-h/cm², and wherein the total area specific impedance for the cell isless than about 20 Ω-cm².

In another aspect of the invention, a low fade lithium secondary cell isproviding having a lithium-containing positive electrode, the positiveelectrode in electronic contact with a positive electrode currentcollector, the current collector in electrical connection with anexternal circuit, a negative electrode in electronic contact with anegative electrode current collector, the current collector inelectrical connection with an external circuit, a separator positionedbetween and in ionic contact with the cathode and the anode, and anelectrolyte in ionic contact with the positive and negative electrodes,wherein the total area specific impedance for the cell and the relativearea specific impedances for the positive and negative electrodes aresuch that the cell is capable of achieving at least about 80% state ofcharge within about 25 minutes, and wherein the cell is capable ofmultiple charge/discharge cycles with a capacity loss of less than about0.2% per cycle.

An aspect of the invention also includes a secondary lithium batteryincluding a positive electrode including a particulate conductiveadditive and a lithium transition metal phosphate having an olivinestructure, the positive electrode having a specific surface area ofgreater than 10 m²/g and a total pore volume between about 40% and about60% by volume, the positive electrode forming a layer on a positiveelectrode current collector having a thickness of about 50 μm to about125 μm, a negative electrode including a particulate conductive additiveand graphitic carbon, the graphitic carbon having an average particlesize of less than about 25 μm, the negative electrode having a totalpore volume between about 25 and 40% by volume and forming a layer on anegative electrode current collector having a thickness of about 20 μmto about 75 μm, a microporous electronically insulating high rateseparator disposed between and in ionic contact with the cathode and theanode, and an electrolyte in ionic contact with the anode and thecathode, wherein the total area specific impedance for the cell and therelative area specific impedances for the positive and negativeelectrodes are such that, during charging at greater than or equal to4C, the negative electrode potential is above the potential of metalliclithium.

Another aspect of the invention is a method of charging a lithiumsecondary cell. The method includes(a) providing a lithium secondarycell including a high capacity lithium-containing positive electrode inelectronic contact with a positive electrode current collector, thecurrent collector in electrical connection with an external circuit, ahigh capacity negative electrode in electronic contact with a negativeelectrode current collector, the current collector in electricalconnection with an external circuit, a separator positioned between andin ionic contact with the cathode and the anode, and an electrolyte inionic contact with the positive and negative electrodes, wherein thetotal area specific impedance for the cell and the relative areaspecific impedances for the positive and negative electrodes are suchthat, during charging at greater than or equal to 4C, the negativeelectrode potential is above the potential of metallic lithium, and (b)charging the cell at a C-rate of at least 4C, wherein at least 95% stateof charge is obtained in less than 15 minutes.

In one or more embodiments, the area specific impedance of the totalcell is localized predominantly at the positive electrode.

In one or more embodiments, the charge capacity per unit area of thepositive and negative electrodes each are at least 0.75 mA-h/cm², or atleast 1.0 mA-h/cm², or at least 1.5 mA-h/cm².

In one or more embodiments, the total area specific impedance for thecell is less than about 16 Ω-cm², or less than about 14 Ω-cm², or lessthan about 12 Ω-cm², or less than about 10 Ω-cm², or less than or equalto about 3.0 Ω-cm².

In one or more embodiments, the total area specific impedance for thecell is less than about 20 Ω-cm², and the positive electrode has an areaspecific impedance r₁ and the negative electrode has an area specificimpedance r₂, and wherein the ratio of r₁ to r₂ is at least about 10, orthe ratio of r₁ to r₂ is at least about 7, or the ratio of r₁ to r₂ isat least about 6, or the ratio of r₁ to r₂ is at least about 5, or theratio of r₁ to r₂ is at least about 4, or the ratio of r₁ to r₂ is atleast about 3.

In one or more embodiments, the negative electrode has an area specificimpedance, r₂, of less than or equal to about 2.5 Ω-cm², or less than orequal to about 2.0 Ω-cm², or less than or equal to about 1.5 Ω-cm².

In one or more embodiments, the positive electrode has a charge anddischarge capacity measured at a C-rate of 10C that is greater than 90%of the nominal capacity measured at a C-rate of 1/10C.

In one or more embodiments, the conductivity of the positive electrodedoes not increase more than a factor of 2 over the state of charge, or afactor of 5 over the state of charge.

In one or more embodiments, the electroactive material of the positiveelectrode is a lithium transition metal phosphate, and the transitionmetal of the lithium transition metal phosphate includes one or more ofvanadium, chromium, manganese, iron, cobalt and nickel. The lithiumtransition metal phosphate is of the formula (Li_(1-x)Z_(x))MPO₄, whereM is one or more of vanadium, chromium, manganese, iron, cobalt andnickel, and Z is one or more of titanium, zirconium, niobium, aluminum,or magnesium, and x ranges from about 0.005 to about 0.05, wherein Z isselected from the group consisting of zirconium and niobium.

In one or more embodiments, the position electrode has a specificsurface area of greater than about 10 m²/g, or greater than about 15m²/g, or greater than about 20 m²/g, or greater than about 30 m²/g. Theposition electrode has pore volume in the range of about 40% to about70% by volume and a thickness in the range of about 50 μm to about 125μm.

In one or more embodiments, the negative electrode includes carbon, suchas graphitic carbon. The carbon is selected from the group consisting ofgraphite, spheroidal graphite, mesocarbon microbeads and carbon fibers.The carbon exhibits fast diffusion direction parallel to the longdimension of the particle with a dimension less than 6*(diffusioncoefficient of fast direction/diffusion coefficient of MCMB)^(0.5) and athickness less than about 75 microns and a porosity greater than 25%.

In one or more embodiments, the carbon of the negative electrode has anaverage particle size of less than about 25 μm, or less than about 15μm, or less than about 10 μm, or less than about 6 μm. The negativeelectrode has pore volume in the range of about 20 and 40% by volume,and a thickness in the range of about 20 μm to about 75 μm.

In one or more embodiments, the cell is charged at a C-rate of 10C,wherein at least 90% state of charge to obtain in less than 6 minutes,or the cell is charged at a C-rate of 20C, wherein at least 80% state ofcharge to obtain in less than 3 minutes. In one or more embodiments, thecell is charged at an overpotential, and the overpotential is apotential near the oxidation potential of the electrolyte.

In one or more embodiments, the cell is capable of achieving at leastabout 90% state of charge within about 12 minutes, and the cell iscapable of multiple charge/discharge cycles with a capacity loss of lessthan about 0.1% per cycle.

In one or more embodiments, the cell is capable of achieving at leastabout 95% state of charge within about 6 minutes, and the cell iscapable of multiple charge/discharge cycles with a capacity loss of lessthan about 0.05% per cycle.

As used herein, the electrical resistivity or impedance, e.g., totalopposition that a battery offers to the flow of alternating current, isgiven in units of ohm, charge and discharge capacity in units of amperehours per kilogram of the storage material (Ah/kg) or milliampere hourper gram of storage material (mAh/g), charge and discharge rate in unitsof both milliamperes per gram of the storage compound (mA/g), and Crate. When given in units of C rate, the C rate is defined as theinverse of the time, in hours, necessary to utilize the full capacity ofthe battery measured at a slow rate. A rate of 1C refers to a time ofone hour; a rate of 2C refers to a time of half an hour, a rate of C/2refers to a time of two hours, and so forth. Typically, the C rate iscomputed from the rate, in mA/g, relative to the capacity of thecompound or battery measured at a lower rate of C/5 or less. “State ofcharge” (SOC) refers to the proportion of the active material stillunused according to Faraday's Law. In the case of a battery, it is theproportion of the cell's capacity that is still unused, with respect toits nominal or rated capacity. A fully-charged battery has SOC=1 or100%, whereas a fully-discharged battery has SOC=0 or 0%. Area specificimpedance (ASI) refers to the impedance of a device normalized withrespect to surface area and is defined as the impedance measured at 1kHz (Ω), using an LCZ meter or frequency response analyzer, multipliedby the surface area of opposing electrodes (cm²).

BRIEF DESCRIPTION OF THE DRAWING

A more complete appreciation of the present invention and many of itsadvantages will be understood by reference to the following detaileddescription when considered in connection with the following drawings,which are presented for the purpose of illustration only are notintended to limit the scope of the appended claims, and in which:

FIG. 1 is a schematic illustration of the local potential (voltage) atvarious locations across the normalized thickness of the cell during lowand high-rate charge cycles in a lithium-ion cell;

FIG. 2 shows a schematic of the electrode potentials during low andhigh-rate charge cycles in a LiCoO₂-graphite anode cell; note that theanode potential drops below 0 V vs Li/Li+, the lithium platingpotential, during high rate charge;

FIG. 3 shows a schematic of the electrode potentials during low andhigh-rate charge cycles in a LiFePO₄-graphite anode cell; note that theanode potential does not drop below 0 V vs Li/Li+, the lithium platingpotential, during the charging cycle;

FIG. 4 is a cross-sectional view showing an exemplary lithium secondarycell having spirally wound electrodes;

FIG. 5 illustrates voltage profile curves in a reference electrodeduring charge at 2C, 5C, 10C and 20C for a lithium ion test cellconstructed according to one or more embodiments of the presentinvention;

FIG. 6 shows the charge and discharge voltage and capacity of a testcell constructed according to one or more embodiments of the presentinvention during extended cycling at a 10C charge and 10C dischargerate; and

FIG. 7 is a plot of capacity vs. cycle number at differentcharge/discharge rates for a commercially available comparativelithium-ion battery.

DETAILED DESCRIPTION OF THE INVENTION

New battery applications demand continuous improvements in batterydischarge rate capabilities and a parallel decrease in charge times.However, when a conventional Li-ion battery is charged at a relativelyhigh rate, e.g., greater than 2C, a decrease in the negative electrodepotential due to impedance brings the negative electrode below thepotential at which lithium plating occurs. This voltage drop may be dueto ohmic resistance, concentration polarization, charge transferresistance, and other sources of impedance.

This phenomenon is illustrated in FIG. 1, which is a schematicillustration of the local potential (voltage) at various locationsacross the normalized thickness of a conventional lithium-ion cell. Thelocations of the positive electrode, separator and negative electrodeare indicated. A series of curves indicates the potential for differentillustrative charge rates. Arrows in the figure indicate the trend forincreasing rate. As the battery is charged at higher rates, the positiveelectrode potential is pushed to a higher potential and the negativeelectrode drops to a lower potential. At high rates, the potential atthe negative electrode drops to below 0 V vs. Li/Li⁺ and plating oflithium metal at the negative electrode occurs. Note that the potentialof the separator changes little over a wide range of charge rates.

During a high rate-constant current charge, total cell voltage isincreased to allow the high charging current to be accommodated. If thecell has high impedance, it must be driven at a higher voltage toachieve the same current flow. FIG. 2 is a schematic illustration of thepositive and negative electrode potentials of a conventional LiCoO₂(“LCO”)-graphite cell, which has a relatively high impedance (ca. 40Ω-cm²) over the entire state of charge. At low charge rates, thenegative electrode potential remains above the lithium platingpotential. During high rate discharge, however, the negative electrodepotential is driven so low that the negative potential drops below thelithium plating potential (0 V vs Li/Li⁺). Lithium plating at the anodetakes place under the conditions indicated by the arrow in FIG. 2.Clearly, the high rate-constant current charge of a high-impedance cellresults in the undesirable plating of lithium.

The advantages of the present invention are illustrated by the lowimpedance Li-ion cell of FIG. 3. In the case of a low-impedance cellaccording to one or more embodiments of the present invention, thenegative electrode does not plate lithium. FIG. 3 shows the positive andnegative electrode potentials for a LiFePO₄ (“LFP”)-graphite cell withan exemplary total area specific impedance (ASI_(tot)) of about 12Ω-cm². During the entire high rate-constant current charging of theLiFePO₄-graphite cell, the potential at the negative anode remains abovethe potential of lithium metal

The positive and negative electrodes represent the greatest contributionto the total area specific impedance (ASI_(tot)) of the cell. Theimpedance of the separator, and the various connecting metal parts ofthe cell such as the tabs, the current collector foils or grids and theelectrode-current collector interfacial resistance generally contributebetween about 10–20%, and typically about 15%, of the total areaspecific impedance (ASI_(tot)).

According to one or more embodiments, the impedance of the negativeelectrode is at a minimum. In a typical Li-ion cell according to one ormore embodiment, the area specific impedance of the negative electrode(ASI_(a)) is less than about 3.0 Ω-cm², or less than about 2.5 Ω-cm², orless than 2.0 Ω-cm², or less than 1.8 Ω-cm², or less than 1.5 Ω-cm².

A further feature of a high rate, low impedance Li-ion cell is that thepositive electrode bears a predominant amount or even a major amount ofthe total cell impedance (ASI_(tot)). In one or more embodiments, up to70% of the cell impedance is localized at the positive electrode. Inparticular, the ratio of area specific impedance of the positiveelectrode (ASI_(c)) to the area specific impedance of the negativeelectrode (ASI_(a)) is greater than about three. In other embodiments,the ratio of area specific impedance of the positive electrode (ASI_(c))to the area specific impedance of the negative electrode (ASI_(a)) is ina range of about 3–10, or is greater than about 4, greater than about 5,greater than about 6, greater than about 7, greater than about 8,greater than about 9, or greater than about 10.

The total area specific impedance of the cell (ASI_(tot)) is less than20 Ω-cm². The total area specific impedance (ASI_(tot)) can be less than18 Ω-cm², or less than 16 Ω-cm², or less than 14 Ω-cm², or less than 12Ω-cm², or less than 10 Ω-cm² or less than 8 Ω-cm². The smaller the valuefor the total area specific impedance (ASI_(tot)), the smaller theproportion of the total impedance required to be borne at the positiveelectrode in order to prevent lithium plating. Table 1 lists anexemplary relationship between total area specific impedance (ASI_(tot))and the area specific impedance at the positive electrode (ASI_(c)) foran exemplary Li-ion cell according to one or more embodiments of thepresent invention.

TABLE 1 ASI_(tot) (Ω-cm²) 8 10 12 14 16 18 20 ASI_(c)/ASI_(a) 3 4 5 6 79 10

Surprisingly, Li-ion cells according to one or more embodiments of thepresent invention achieve high charge rates in cells having thickelectrode layers, e.g., a positive electrode layer of about 50 μm toabout 125 μm on one side of the current collector. While thickerelectrode layers provide higher charge capacity, the thicker layers alsotypically increase the impedance of the electrodes (by, for example,increasing the distance and the tortuosity of the lithium diffusionpathway). In a single cell consisting of a positive and negativeelectrode in ionic contact with one another through the electrolyte, theareal charge capacity is one-half of the measured areal capacity for thedouble-sided electrode, e.g., at least 0.75 mA-hr/cm². It has beensurprisingly discovered that a Li-ion cell having areal chargecapacities of at least 0.75 mA-hr/cm², or 1.0 mA-h/cm² or 1.5 mA-hr/cm²are capable of high rate charge and discharge without plating lithium atthe negative electrode.

A prior art method of obtaining a high charge and discharge rates is toreduce the areal capacity of the cell, e.g., by using very thinelectrodes. A very thin electrode (i.e., with a low areal capacity)could achieve high charge and discharge capacity at high rates; however,the low mass/volume of the electrode in the cell would not result in apractical device. The cell according to one or more embodiments of thepresent invention provides both high rate capability AND high chargecapacity.

In one or more embodiments of the present invention, a high capacityLi-ion cell is charged and discharged at a high rate, e.g., greater than2C, greater than 4C, or greater than 10C, or even at 20C, withoutsignificant capacity fade. The cell can be initially charged by thegalvanostatic (constant current) method to target voltage, e.g., 3.6–3.8V for a LiFePO₄-C cell, using a high C-rate (2, 5, 10, and 20C.) Afterthe target voltage is reached, a potentiostatic segment can be applieduntil the current decreases to a C/20 rate (CC-CV protocol or tapercharge method), which is considered to be ‘fully charged’ or state ofcharge. The time to achieve state of charge is very fast, e.g., lessthan 15 minutes, with low levels of cell heating. This can be comparedto a low charge rate of 1C, requiring 60 minutes for state of charge.

The inventors have found that the batteries made according to thepresent invention show surprisingly low fade rate when charged at a highrate. For batteries charged at 10C, high capacity lithium-ion cells showless than 0.2% loss per cycle, 0.1% loss per cycle, 0.05% loss percycle, and 0.025% loss per cycle.

In one or more embodiments, the Li-ion cell charges at 4C-rate andreaches 90%, or even 95%, state of charge within 15 minutes. OtherLi-ion cells charge at 10C-rate and achieve 80%, or even 90%, state ofcharge within 6 minutes. The Li-ion cells also possess superiordischarge rate capabilities as compared to conventional Li-ion cells.Li-ion cells according to one or more embodiments of the presentinvention demonstrate 10C capacity of greater than 70%, or 80%, or 90%,or even 95% of nominal capacity measured at C/10.

In another embodiment of the present invention, the lithium-ion batterycan be charged to potentials well above the standard charging potential,in order to charge the battery more quickly. In a conventional 4.2Vlithium-ion battery, such as one that contains LiCoO₂, the maximumcharging current is also limited by the potential at the positiveelectrode. A high potential at the positive electrode will causeelectrolyte oxidation, which greatly decreases the lifetime of thebattery. Lithium iron phosphate has a lower average voltage duringcharge. Thus, a positive electrode incorporating lithium iron phosphateas the active material can be polarized to a greater extent beforereaching the electrolyte oxidation potential.

In a preferred embodiment of the present invention, transition metalphosphate positive electrode materials are charged using anoverpotential because there is no instability in the delithiated state.As a result, there is no excess lithium. In contrast, conventionalpositive electrode materials, using LiCoO₂ for example, cannot becharged to potentials greater than 4.2V because of its instability inthe delithiated state. The larger overpotential at the positiveelectrode, i.e., the potential above the standard charging potential,allows the cell to be charged at a high, constant current for a longerperiod of time before the charging current must be decreased or beforethe cell is placed on a potentiostatic, or constant voltage, hold. Thus,the cell can be charged more quickly without danger of electrolyteoxidation. The lower average voltage of the positive electrode materialis particularly useful when combined with a low-impedance negativeelectrode (or a higher positive electrode-to-negative electrodeimpedance ratio (ASI_(c)/ASI_(a))), as described in herein. Note that ahigh impedance negative electrode would not be useful because lithiumwould plate onto the anode regardless of the positive electrodepotential.

Typically, the rate capability of a cell is determined by a constantcurrent or constant power continuous discharge, which gives rise to aRagone plot. In one embodiment of this invention, the discharge energydensity of the battery is 85 Wh/kg at a power density of 750 W/kg.Ragone plots are used to describe energy density during discharge, notcharge. So other methods are used to describe the high charge capabilityof this invention.

According to one or more embodiments, a Li-ion cell is provided forwhich the resistance of the components contributing to the voltage dropat the negative electrode are minimized. Factors affecting the impedance(and hence rate capability) at the negative electrode itself duringhigh-rate discharge include electrode thickness, bulk electronicconductivity, contact resistance between current collector and activematerial particles, average size of active material—typicallycarbon—particles, Li⁺ diffusion coefficient in the active material,electrode porosity, pore size distribution and tortuosity, ionicconductivity of the liquid electrolyte, and transference number of Li⁺in the liquid electrolyte. The factors listed above that strongly affectthe negative electrode's rate capability are equally important in thecase of the positive electrode as well.

A Li-ion battery capable of safe and long-term operation at a high rateof charge and discharge without a significant loss of power and capacityand a method of its manufacture according to the present invention isdescribed in detail. The positive and negative electrodes are designedat the (1) active particle level, (2) electrode level, and (3) celllevel to maximize rate, reduce impedance, in particular at the negativeelectrode, while maintaining a high charge capacity.

The nonaqueous electrolyte secondary battery includes a battery elementhaving an elongated cathode and an elongated anode, which are separatedby two layers of an elongated microporous separator which are tightlywound together and placed in a battery can. A typical spiral electrodesecondary cell is shown in FIG. 4 (reproduced from U.S. Pat. No.6,277,522). The secondary cell 15 includes a double layer of anodematerial 1 coated onto both sides of an anode collector 10, a separator2 and a double layer of cathode material 3 coated onto both sides ofcathode collector 11 that have been stacked in this order and wound tomake a spiral form. The spirally wound cell is inserted into a batterycan 5 and insulating plates 4 are disposed at upper and lower surfacesof the spirally wound cell. A cathode lead 13 from anode collector 11provides electrical contact with cover 7. An anode lead 12 is connectedto the battery can 5. An electrolytic solution is added to the can.

A Li-ion battery capable of safe, long-term operation at a high rate ofcharge and discharge and a method of its manufacture includes one ormore of the following features.

At the material level, the positive electrode includes alithium-transition metal-phosphate compound as the electroactivematerial. The lithium-transition metal-phosphate compound may beoptionally doped with a metal, metalloid, or halogen. The positiveelectroactive material can be an olivine structure compound LiMPO₄,where M is one or more of V, Cr, Mn, Fe, Co, and Ni, in which thecompound is optionally doped at the Li, M or O-sites. Deficiencies atthe Li-site are compensated by the addition of a metal or metalloid, anddeficiencies at the O-site are compensated by the addition of a halogen.In some embodiments, the positive active material is a thermally stable,transition-metal-doped lithium transition metal phosphate having theolivine structure and having the formula (Li_(1-x)Z_(x))MPO₄, where M isone or more of V, Cr, Mn, Fe, Co, and Ni, and Z is a non-alkali metaldopant such as one or more of Ti, Zr, Nb, Al, or Mg, and x ranges from0.005 to 0.05. In a typical battery, the electroactive material is(Li_(1-x)Z_(x))MPO₄, where Z is Zr or Ti.

Doped lithium iron phosphate compounds may be prepared from startingmaterials of lithium salts, iron compounds and phosphorous saltsincluding, but not limited to, lithium carbonate, ammonium phosphate andiron oxalate, to which a low additional concentration of dopant metalsuch as Mg, Al, Ti, Fe, Mn, Zr, Nb, Ta and W have been added, typicallyas a metal oxide or metal alkoxide. The powder mixture is heated under alow oxygen environment at a temperature of 300° C. to 900° C. Thesecompounds exhibit increased electronic conductivity at and near roomtemperature, which is particularly advantageous for their use as lithiumstorage materials. Further details regarding the composition andpreparation of these compounds are found in U.S. Published application2004/0005265, which is incorporated herein in its entirety by reference.

The transition-metal doped LiFeO₄ has a markedly smaller particle sizeand much larger specific surface area than previously known positiveactive materials, such as LiCoO₂, LiNiO₂ or LiMn₂O₄ and, thus improvedtransport properties. In some embodiments the positive active materialconsists of powder or particulates with a specific surface area ofgreater than 10 m²/g, or greater than 15 m²/g, or greater than 20 m²/g,or even greater than 30 m²/g. While methods are known to produce thesetraditional positive active materials in the form of high specificsurface area powders, Li-ion battery batteries made from such materialshave inferior safety and stability characteristics due to a combinationof the high oxidation potential and low inherent thermal stability ofthese conventional materials in their partially or fully delithiatedform, such as that existing in a partially or fully charged Li-ionbattery.

The present inventors have unexpectedly discovered that LiFeO₄ havingthe olivine structure and made in the form of very small, high specificsurface area particles are exceptionally stable in their delithiatedform even at elevated temperatures and in the presence of oxidizableorganic solvents, e.g., electrolytes, thus enabling a safer Li-ionbattery having a very high charge and discharge rate capability. Theinventors have also found that the small-particle-size, highspecific-surface-area LiFePO₄ material exhibits not only high thermalstability, low reactivity and high charge and discharge rate capability,but it also exhibits excellent retention of its lithium intercalationand deintercalation capacity during many hundreds, or even thousands, ofhigh-rate cycles.

On an electrode level, the active material and a conductive additive arecombined to provide an electrode layer that permits rapid lithiumdiffusion throughout the layer. A conductive additive such as carbon ora metallic phase is included in order to improve its electrochemicalstability, reversible storage capacity, or rate capability. Exemplaryconductive additives include carbon black, acetylene black, vapor grownfiber carbon (“VGCF”) and fullerenic carbon nanotubes. Conductivediluents are present in a range of about 1%–5% by weight of the totalsolid composition of the positive electrode.

The positive electrode (cathode) is manufactured by applying asemi-liquid paste containing the cathode active compound and conductiveadditive homogeneously dispersed in a solution of a polymer binder in anappropriate casting solvent to both sides of a current collector foil orgrid and drying the applied positive electrode composition. A metallicsubstrate such as aluminum foil or expanded metal grid is used as thecurrent collector. To improve the adhesion of the active layer to thecurrent collector, an adhesion layer, e.g., thin carbon polymerintercoating, may be applied. The dried layers are calendared to providelayers of uniform thickness and density. The binder used in theelectrode may be any suitable binder used as binders for non-aqueouselectrolyte cells. Exemplary materials include a polyvinylidene fluoride(PVDF)-based polymers, such as poly(vinylidene fluoride) (PVDF) and itsco- and terpolymers with hexafluoroethylene, tetrafluoroethylene,chlorotrifluoroethylene, poly(vinyl fluoride), polytetraethylene (PTFE),ethylene-tetrafluoroethylene copolymers (ETFE), polybutadiene,cyanoethyl cellulose, carboxymethyl cellulose and its blends withstyrene-butadiene rubber, polyacrylonitrile, ethylene propylene dieneterpolymers (EPDM), styrene-butadiene rubbers (SBR), polyimides,ethylene-vinyl acetate copolymers.

The positive electrode containing the positive electroactive materialhas a specific surface area of the electrode measured using the nitrogenadsorption Brunauer-Emmett-Teller (BET) method after the densificationor calendaring step that is greater than 10 m²/g or greater than 20m²/g. A positive electrode can have a thickness of less than 125 μm,e.g., between about 50 μm to 125 μm, or between about 80μm to 100 μm oneach side of the current collector, and a pore volume fraction betweenabout 40 and 70 vol. %. The active material is typically loaded at about10–20 mg/cm², and typically about 11–15 mg/cm². In general, a thickerelectrode layer (and higher active material loading) provides greatertotal capacity for the battery. However, thicker layers also increasethe electrode impedance. The present inventors have surprisinglydiscovered that high capacity, thick layers may be used in a lowimpedance (high rate) cell. Use of a high specific surface area activematerial, while maintaining adequate pore volume, provides the desiredcapacity without increasing impedance to unacceptably high levels.

In another embodiment of the present invention, the electroactivematerial of the positive electrode includes a material that, while ofhigh electronic conductivity, does not vary its conductivity by morethan a factor of five, or factor of two, over the entire charge cycle.This feature of the Li-ion cell is contrasted with conventionalelectroactive positive electrode materials such as LiCoO₂, LiNiO₂ orLiMn₂O₄ for which conductivity increases dramatically once delithiationduring charging occurs. The dramatic increase in conductivity of theelectroactive material of the positive electrode contributes to adecrease in impedance. In contrast, an electroactive material of thepresent cells exhibit only moderate increases in conductivity, so thatits contribution to impedance is more moderate.

The selection criteria for an anode are at two levels, the particlelevel and the electrode level. At the particle level, the particle sizeand the Li diffusion coefficient of the particle are selection criteria.In one embodiment, the negative active material is a carbonaceousmaterial. The carbonaceous material may be non-graphitic or graphitic. Asmall-particle-size, graphitized natural or synthetic carbon can serveas the negative active material. Although non-graphitic carbon materialsor graphite carbon materials may be employed, graphitic materials, suchas natural graphite, spheroidal natural graphite, mesocarbon microbeadsand carbon fibers, such as mesophase carbon fibers, are preferably used.The carbonaceous material has a numerical particle size (measured by alaser scattering method) that is smaller than about 25 μm, or smallerthan about 15 μm, or smaller than about 10 μm, or even less than orequal to about 6 μm. The smaller particle size reduces lithium diffusiondistances and increases rate capability of the anode, which is a factorin preventing lithium plating at the anode. In those instances where theparticle is not spherical, the length scale parallel to the direction oflithium diffusion is the figure of merit. Larger particle sizedmaterials may be used if the lithium diffusion coefficient is high. Thediffusion coefficient of MCMB is ˜10e-10 cm²/s. Artificial graphite hasa diffusion coefficient of ˜10e-8 cm²/s. As a result larger particlesize artificial graphite could be used, approximately equal to 15microns times the square root of the ratio of the respectivediffusivities (H. Yang et al., Journal of Electrochemical Society, 151(8) A1247–A1250 (2004)).

In some embodiments, the negative active material consists of powder orparticulates with a specific surface area measured using the nitrogenadsorption Brunauer-Emmett-Teller (BET) method to be greater than about2 m²/g, or 4 m²/g, or even about 6 m²/g.

On an electrode level, the active material and a conductive additive arecombined to provide an electrode layer that permits rapid lithiumdiffusion throughout the layer. A conductive additive such as carbon ora metallic phase may also be included in the negative electrode.Exemplary conductive additives include carbon black, acetylene black,vapor grown fiber carbon (“VGCF”) and fullerenic carbon nanotubes.Conductive diluents are present in a range of about 0%–5% by weight ofthe total solid composition of the negative electrode.

The negative electrode (anode) of the battery is manufactured bypreparing a paste containing the negative active material, such asgraphitic or non-graphitic carbon, and a conductive carbon additivehomogeneously suspended in a solution of a polymer binder in a suitablecasting solvent. The paste is applied as a uniform-thickness layer to acurrent collector and the casting solvent is removed by drying. Ametallic substrate such as copper foil or grid is used as the negativecurrent collector. To improve the adhesion of the active material to thecollector, an adhesion promoter, e.g., oxalic acid, may be added to theslurry before casting. The binder used in the negative electrode may beany suitable binder used as binders for non-aqueous electrolyte cells.Exemplary materials include a polyvinylidene fluoride (PVDF)-basedpolymers, such as poly(vinylidene fluoride) (PVDF) and its co- andterpolymers with hexafluoroethylene, tetrafluoroethylene,chlorotrifluoroethylene, poly(vinyl fluoride), polytetraethylene (PTFE),ethylene-tetrafluoroethylene copolymers (ETFE), polybutadiene,cyanoethyl cellulose, carboxymethyl cellulose and its blends withstyrene-butadiene rubber, polyacrylonitrile, ethylene propylene dieneterpolymers (EPDM), styrene-butadiene rubbers (SBR), polyimides,ethylene-vinyl acetate copolymers.

At the electrode level, the negative electrode can have a thickness ofless than 75 μm, e.g., between about 20 μm to 65 μm, or between about 40μm to 55 μm on both sides of the current collector, and a pore volumefraction between about 20 and 40 vol. %. The active material istypically loaded at about 5–20 mg/cm², or about 4–5 mg/cm². In general,a thicker electrode layer (and higher active material loading) providesgreater total capacity for the battery. However, thicker layers alsoincrease the electrode impedance by reducing the ease of lithiumdiffusion into the anode. The present inventors have surprisinglydiscovered that high capacity, thick layers may be used in a lowimpedance cell through selection of active materials as indicated aboveand maintaining adequate pore volume.

A nonaqueous electrolyte is used and includes an appropriate lithiumsalt dissolved in a nonaqueous solvent. The electrolyte may be infusedinto a porous separator that spaces apart the positive and negativeelectrodes. In one or more embodiments, a microporous electronicallyinsulating separator is used.

Numerous organic solvents have been proposed as the components of Li-ionbattery electrolytes, notably a family of cyclic carbonate esters suchas ethylene carbonate, propylene carbonate, butylene carbonate, andtheir chlorinated or fluorinated derivatives, and a family of acyclicdialkyl carbonate esters, such as dimethyl carbonate, diethyl carbonate,ethylmethyl carbonate, dipropyl carbonate, methyl propyl carbonate,ethyl propyl carbonate, dibutyl carbonate, butylmethyl carbonate,butylethyl carbonate and butylpropyl carbonate. Other solvents proposedas components of Li-ion battery electrolyte solutions include γ-BL,dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran,1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane,methylsulfolane, acetonitrile, propiononitrile, ethyl acetate, methylpropionate, ethyl propionate and the like. These nonaqueous solvents aretypically used as multicomponent mixtures.

A solid or gel electrolyte may also be employed. The electrolyte may bean inorganic solid electrolyte, e.g., LiN or LiI, or a high molecularweight solid electrolyte, such as a gel, provided that the materialsexhibits lithium conductivity. Exemplary high molecular weight compoundsinclude poly(ethylene oxide), poly(methacrylate) ester based compounds,or an acrylate-based polymer, and the like.

As the lithium salt, at least one compound from among LiClO₄, LiPF₆,LiBF₄, LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂CF₂CF₃)₂ and the like are used.The lithium salt is at a concentration from 0.5 to 1.5 M, or about 1.3M.

The above described positive electrode is brought into intimate contactwith the negative electrode through the separator layers, which are thenspirally wound a number of times around a small-diameter mandrel to formthe jelly-roll electrode-separator assembly. Next, the jelly-rollstructure is inserted into a nickel-plated steel battery can, currentcollector tabs are spot-welded to the battery can and can header, whichis preferably equipped with a variety of safety features, such aspositive-temperature coefficient elements, pressure burst disks, etc.Alternatively, uncoated regions can be created along the edge of theelectrode, thereby exposing bare metal foil. One or preferably moremetal foil strips or tabs, between 0.4 and 0.8 cm wide, can be attachedto these bare regions using an ultrasonic welder. These tabs can then beattached to the can or header using an ultrasonic or spot (resistance)welder. The nonaqueous electrolyte including a solution of a lithiumsalt in a mixture of carbonate esters is injected into the battery can,the can header is sealed to the battery can using a crimp seal or laserweld.

According to one or more embodiments, a Li-ion battery contains anoptionally doped lithium transition metal phosphate positive electrode,a highly microporous electronically insulating separator layer, agraphitized-carbon negative electrode, and a multicomponent liquidorganic electrolyte solution in which a lithium salt is dissolved at aconcentration from 0.5 to 1.5 M. Both the positive and negativeelectrodes have high surface area and high pore volume. In order toreduce the chance of lithium plating at the anode, the lithium capacityof the negative electrode is higher than that of the positive electrode.The battery is capable of being charged and discharged at a very highrate, due to having the above described relative electrode resistances,which is accomplished by the selection of appropriate active materials,e.g., composition, particle size, porosity, surface area, pore volume,etc., and by the addition of appropriate amounts of conductive diluentssuch as carbon to the positive or negative electrode. The types,amounts, and methods of adding such conductive diluents are readilydetermined by methods well-known to those skilled in the art.

Although the particular embodiment of a Li-ion battery described hererelates to a cylindrical cell, it is to be understood that the presentinvention is not limited to such a battery shape. In fact, other canshapes and sizes, such as square, rectangular (prismatic) coin, buttonor the like may be used.

Further, although the above description uses an example of a liquid typenonaqueous electrolyte Li-ion battery, it is to be understood that othertypes of non-aqueous electrolytes, such as those of gel or solid polymertype can be used to manufacture thin batteries of this invention, whoseelectrodes may be bonded to their respective separators and packaged inthin metal-polymer laminate film bags as an outer casing material.

EXAMPLES Example 1

Preparation of a Lithium-Ion Secondary Cell

To prepare doped LiFePO₄, iron oxalate, lithium carbonate, ammoniumdihydrogen phosphate and zirconium ethoxide are mixed in a 2:1:2:0.02molar ratio in a plastic milling jar containing grinding media andacetone for 72 hours. Heating and stirring the slurry to the boilingpoint of acetone remove the acetone. The dried powder is heated under aninert atmosphere at 1° C. per minute to 350° C. and held there for 10hours, followed by ramping at 5 degrees per minute to 600° C. andholding there for 20 hours. The finished product is milled and thenstored in the absence of water.

The positive electrode slurry is prepared by dissolving 7 g of PVDF-HFPcopolymer commercially available as Kynar® 2801 from AtoFina in 250 g ofNMP and dispersing in the resulting solution a dry mixture of 88 g ofdoped LiFeO₄ prepared as described above and 5 g of conductive carbon(Super P or Ensaco). The paste is homogenized in a planetary mixer orblender, cast on both sides of an aluminum foil current collector usinga die casting apparatus, dried in an oven to remove the casting solventand densified using a calendering apparatus. The electrode mass thusprepared was carefully scraped from the current collector foil and itsporosity determined to be 53–57 vol.-%. Its specific surface areadetermined by the BET method was 22–25 m²/g. The two-sided thickness ofthe calendered positive electrode, including current collector foil, wasapproximately 200 μm. The positive electrode had an areal capacity ofapproximately 1.6 mAh/cm².

The negative electrode was prepared by dissolving 8 g of PVDF-HFPcopolymer described above in 250 ml of NMP, adding to it a mixture of 88g of mesophase microbead synthetic graphitic carbon MCMB 6-28 (Osaka GasCo., Ltd.) and 4 g of conductive carbon (Super P). The paste washomogenized in a planetary mixer or blender, cast on both sides of acopper current collector foil using a die casting apparatus, dried in anoven and densified using a calendering apparatus. The negative electrodeporosity was determined to be 29–33 vol.-%. The two-sided thickness ofthe calendered negative electrode, including current collector foil, wasapproximately 90 μm. The negative electrode had an areal capacity ofapproximately 1.7 mAh/cm².

Both electrodes were cut to proper dimensions, interposed with aslightly larger elongated pieces of a microporous polyolefin separatorCelgard® 2500 (Celgard LLC), assembled into an 18650-size cylindricalcell by a method well-understood by those schooled in the art andactivated with a 1.3 M solution of LiPF₆ in a mixture of cyclic andacyclic carbonate esters.

Total Cell Areal Specific Impedance Measurement.

Area specific impedance (ASI) is the impedance of a device normalizedwith respect to surface area and is defined as the impedance measured at1 kHz (Ω), using an LCZ meter or frequency response analyzer, multipliedby the surface area of opposing electrodes (cm²). This measurement wasperformed by applying a small (5 mV) sinusoidal voltage to the cell andmeasuring the resulting current response. The resulting response can bedescribed by in-phase and out-of-phase components. The in-phase (real orresistive) component of the impedance at 1 kHz is then multiplied by thesurface area of opposing electrodes (cm²) to give the area specificimpedance. The area specific impedance of the cell from Example 1 was 15Ω-cm².

Example 2

Preparation of a Li-Ion Cell

A positive electrode was prepared as described in Example 1, the onlyexception being that acetone was used instead of NMP as a castingsolvent to prepare a positive electrode paste. A cylindrical Li-ionbattery was assembled exactly following the steps and proceduresdescribed in Example 1. The positive electrode material removed from thecurrent collector foil after calendering had a porosity of 27 vol.-% andspecific surface area of 13 m /g.

Example 3

Preparation of a Li-Ion Cell

A positive electrode was prepared as described in Example 1, the onlyexception being that an acetone-NMP mixture in the volumetric ratio of90 to 1 was used instead of pure NMP as a casting solvent to prepare apositive electrode paste. A cylindrical Li-ion battery was assembledexactly following the steps and procedures described in Example 1.

Example 4

Preparation of a Li-Ion Cell

A negative carbon-based electrode was prepared following the proceduredescribed in Example 1, the only exception being that alarger-particle-size mesophase microbead graphitic-type carbon, MCMB10–28 (Osaka Gas Co., Ltd.) was used instead of MCMB 6–28. A cylindricalLi-ion battery was then assembled exactly following the steps andprocedures described in Example 1.

Example 5

Negative Electrode Area Specific Impedance Measurement

Pouch-type test cells were assembled using rectangular electrode piecespunched out of the positive and negative electrodes described in Example1, with the following exceptions: (1) an acetone-NMP mixture in thevolumetric ratio of 90 to 10 was used, instead of pure NMP as a castingsolvent to prepare a positive electrode paste; (2) Celgard E903, ratherthan Celgard 2500, microporous separator was used; and (3) 1.0 Msolution of LiPF6 in a mixture of cyclic and acyclic carbonate esterswas used as the electrolyte.

After the electrodes were punched to the correct size and shape, aportion of each electrode was removed to reveal bare metal foil. Thisbare metal foil region was approximately two inches long and 0.5 incheswide and served as a tab for current collection. A piece of separatorwas placed between the two electrodes. Then, another small piece ofseparator was used to electrically insulate a small piece of lithiumplaced on the edge of a strip of copper foil. This lithium referenceelectrode was placed between the two previously mentioned electrodes,near the outside edge. The entire assembly was then placed in a thin,metal-polymer laminate film sealed on three sides to create a pouch orbag as an outer casing material. Sufficient electrolyte was added tofully wet the separator and the bag was sealed across the bare metalfoil tabs, using an impulse sealer. The pouch cell was placed betweentwo rigid plates, which were then clamped together using binder clips.

The area specific impedance of each electrode was measuredindependently, according to the method described in Example 1. In thecase of a three electrode cell, the contribution of the anode andcathode impedance to the overall cell impedance can be separated.Measurement of the reference electrode cell showed that the negativeelectrode area specific impedance was less than 2 Ω-cm².

Example 6

Charge/Discharge Cycling of Li-ion Cell at Different C-Rates

A reference electrode pouch cell was fabricated following the proceduredescribed in Example 5.

The cell was initially charged by the galvanostatic (constant current)method to 3.8 V using progressively higher C-rates (2, 5, 10, and 20C.)After each charge, a potentiostatic segment was applied until thecurrent decreased to a C/20 rate (CC-CV protocol or taper chargemethod). The potentials of the positive and negative electrodes wererecorded independently using the lithium reference electrode, which areshown in FIG. 5. In FIG. 5, the positive electrode (cathode) potentialis represented by a dashed line at the top of the figure and thenegative electrode (anode) potential is represented by a heavy line inthe lower portion of the figure. The potential of the anode remainsabove 0 V (the plating potential of lithium metal) even at charge ratesof 20 C. The charging cycle at 10C and 20C is extremely fast. State ofcharge is achieved at very short charge durations, e.g., about 6 minutesat 10C, with low levels of cell heating. This can be compared to a lowcharge rate of IC, requiring 60 minutes for state of charge.

The figure demonstrates that the cell can be charged at rates up to 20 Cwithout plating of lithium at the negative electrode. The positiveelectrode polarization (as indicated by the horizontal arrows in thefigure) is much larger than the negative electrode polarization,indicating that a majority of the impedance in the system occurs at thepositive electrode, thus preventing the negative electrode from reachingthe lithium plating potential.

Example 7

Cycle Life of a Li-ion Cell at 10C

An 18650-type cylindrical cell was assembled using positive and negativeelectrodes as described in Example 1, with the only exception being thatan acetone-NMP mixture in the volumetric ratio of 90 to 10 was used,instead of pure NMP as a casting solvent, to prepare a positiveelectrode paste. The 18650 cylindrical Li-ion battery was assembledexactly following the steps and procedures described in Example 1.

The cell was charged by the galvanostatic (constant current) method to3.8 V at a 10C rate and followed by a potentiostatic segment until thecurrent decreased to a C/20 rate (CC-CV protocol or taper chargemethod). The cell was then discharged at 10C, allowed to rest for 30minutes, then charged again. The data was normalized to the 10C capacityduring the first discharge. FIG. 6 is a plot of discharge capacity vs.cycle number for the cell, demonstrating only a 2.6% capacity loss over98 cycles. This represents a capacity fade of only 0.026% per cycle.

Comparative Example 1

For comparison purposes, a number of contemporary commercial Li-ioncells made by several leading manufacturers were recovered from theirmulti-cell battery packs and subjected to several slow (C/5)charge-discharge cycles between 4.2 and 2.8 V followed by a series ofsingle discharges at discharge rates from C/2 to 4C. The best performingcell type (an 800 mAh prismatic cell based on the LiCoO₂-graphite couplewhich showed very low capacity fade during slow cycling and the highestrate capability (84% capacity retention at a 4C rate)) was selected forfurther comparative testing.

The cell was cycled at a IC rate of charge and a 2C rate of dischargebetween 2.8 and 4.2 V. The cell capacity (measured in units of mA-h)decreased from approximately 660 mA-h to 560 mA-h over 40 cycles, whichrepresents a total decrease in capacity of 15.2% total and a loss incapacity of 0.38% per cycle. A similar cell that was cycled at a 4Ccharge rate and a 2C discharge rate exhibited even poorer capacity fadeperformance. After 50 cycles, the cell exhibited a 42.4% loss ofcapacity, representing 0.85% capacity loss per cycle. Life cycleperformance of these comparative lithium-ion cells is shown in FIG. 8.

Those skilled in the art would readily appreciate that all parametersand configurations described herein are meant to be exemplary and thatactual parameters and configurations will depend upon the specificapplication for which the systems and methods of the present inventionare used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific embodiments of the invention described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, the invention may be practiced otherwise thanas specifically described. Accordingly, those skilled in the art wouldrecognize that the use of an electrochemical device in the examplesshould not be limited as such. The present invention is directed to eachindividual feature, system, or method described herein. In addition, anycombination of two or more such features, systems or methods, if suchfeatures, systems or methods are not mutually inconsistent, is includedwithin the scope of the present invention.

1. A secondary lithium battery, comprising: a positive electrodecomprising a layer on a positive electrode current collector, said layercomprising particulate conductive additive and a lithium transitionmetal phosphate having an olivine structure and having a thickness ofabout 50 μm to about 125 μm, and said positive electrode having aspecific surface area of greater than 10 m²/g and a total pore volumebetween about 40% and about 60% by volume; a negative electrodecomprising a layer on a negative electrode current collector, said layercomprising conductive additive and carbon and having a thickness ofabout 20 μm to about 75 μm, the carbon having an average particle sizeof less than about 25 μm, and said negative electrode having a totalpore volume between about 25 and 40% by volume; a microporouselectronically insulating high rate separator disposed between and inionic contact with the cathode and the anode; and an electrolyte inionic contact with the anode and the cathode, wherein the cell has atotal area specific impedance, and the positive and negative electrodeseach have an area specific impedance and the area specific impedance ofthe positive electrode is greater than the area specific impedance ofthe negative electrode, wherein during charging at greater than or equalto 4C, the negative electrode potential is above the potential ofmetallic lithium.
 2. The secondary lithium battery of claim 1, whereinthe area specific impedance of the total cell is localized predominantlyat the positive electrode.
 3. The lithium secondary battery of claim 1,wherein the current capacity per unit area of the positive and negativeelectrodes each are at least 1.0 mA-h/cm2.
 4. The lithium secondarybattery of claim 1, wherein the current capacity per unit area of thepositive and negative electrodes each are at least 1.5 mA-h/cm2.
 5. Thelithium secondary battery of claim 1, wherein the transition metal ofthe lithium transition metal phosphate comprises one or more ofvanadium, chromium, manganese, iron, cobalt and nickel.
 6. The lithiumsecondary battery of claim 1, wherein the lithium transition metalphosphate is of the formula (Li_(1−x)Z_(x))MPO₄, where M is one or moreof vanadium, chromium, manganese, iron, cobalt and nickel, and Z is oneor more of titanium, zirconium, niobium, aluminum, or magnesium, and xranges from about 0.005 to about 0.05.
 7. The lithium secondary batteryof claim 6, wherein Z is selected from the group consisting of zirconiumand niobium.
 8. The lithium secondary battery of claim 1, wherein thepositive electrode has a specific surface area of greater than about 15m²/g.
 9. The lithium secondary battery of claim 1, wherein the positiveelectrode has a specific surface area of greater than about 20 m²/g. 10.The lithium secondary battery of claim 1, wherein the positive electrodehas a specific surface area of greater than about 30 m²/g.
 11. Thelithium secondary battery of claim 1, wherein the carbon of the negativeelectrode is graphitic.
 12. The lithium secondary battery of claim 11,wherein the graphitic carbon of the negative electrode has an averageparticle size of less than about 10 μm.
 13. The lithium secondarybattery of claim 11, wherein the graphitic carbon of the negativeelectrode has an average particle size of less than about 6 μm.
 14. Thelithium secondary battery of claim 1, wherein the negative electrode hasan area specific impedance, r₂, of less than or equal to about 3.0Ω-cm².
 15. The lithium secondary battery of claim 1, wherein thenegative electrode has an area specific impedance, r₂, of less than orequal to about 2.5 Ω-cm².
 16. The lithium secondary battery of claim 1,wherein the negative electrode has an area specific impedance, r₂, ofless than or equal to about 2.0 Ω-cm².
 17. The lithium secondary batteryof claim 1, wherein the negative electrode has an area specificimpedance, r₂, of less than or equal to about 1.5 Ω-cm².
 18. The lithiumsecondary battery of claim 1, wherein the positive electrode has acharge and discharge capacity measured at a C-rate of 10C that isgreater than 90% of the nominal capacity measured at a C-rate of 1/10C.19. The lithium secondary battery of claim 1, wherein the total areaspecific impedance for the cell is less than about 14 Ω-cm² and whereinthe positive electrode has an area specific impedance r₁ and thenegative electrode has an area specific impedance r₂, wherein the ratioof r₁ to r₂ is at least about
 6. 20. The lithium secondary battery ofclaim 1, wherein the total area specific impedance for the cell is lessthan about 12 Ω-cm² and wherein the positive electrode has an areaspecific impedance r₁ and the negative electrode has an area specificimpedance r₂, wherein the ratio of r₁ to r₂ is at least about
 5. 21. Thelithium secondary battery of claim 1, wherein the total area specificimpedance for the cell and the relative area specific impedances for thepositive and negative electrodes are such that the cell is capable ofachieving at least about 80% state of charge within about 25 minutes,and wherein the cell is capable of multiple charge/discharge cycles witha capacity loss of less than about 0.2% per cycle.
 22. The lithiumsecondary battery of claim 1, wherein the lithium transition metalphosphate comprises a doped lithium transition metal phosphate.
 23. Thelithium secondary battery of claim 22, wherein the doped lithiumtransition metal phosphate comprises a dopant selected from the groupconsisting of metals, metalloids and halogens.
 24. The lithium secondarybattery of claim 22, wherein the doped lithium transition metalphosphate is doped one or more of the Li, M or O-sites.
 25. The lithiumsecondary battery of claim 11, wherein the graphitic carbon of thenegative electrode has an average particle size of less than about 15μm.
 26. The lithium secondary battery of claim 11, wherein the graphiticcarbon is selected from the group consisting of natural graphite,spheroidal natural graphite, mesocarbon microbeads and carbon fibers,such as mesophase carbon fibers.